1. Field of the Invention
The present invention relates to a liquid discharge apparatus and a method for driving a liquid discharge head.
2. Description of the Related Art
A liquid discharge apparatus, for example, an inkjet recording apparatus is used in an image forming apparatus (or, an image recording apparatus) such as a printer, a facsimile apparatus, a copier, or a plotter. The inkjet head (liquid discharge head) in the inkjet recording apparatus includes a nozzle configured to discharge ink droplets, a pressure generation chamber (also referred to, for example, as a pressure chamber, a pressure liquid chamber, a liquid chamber, an ink chamber, or an ink channel) with which the nozzle communicates, an actuator (energy generating unit) configured to pressurize the ink in the pressure generation chamber. The inkjet head pressurizes the ink in the pressure generation chamber by driving the actuator. This discharges the ink droplets from the nozzle opening. An ink on demand inkjet head that discharges the ink droplets only when recording is required is used as such an inkjet head in most cases.
Such inkjet heads are classified into some types according to the actuator configured to generate pressure to discharge the ink droplets (recording liquid).
One of the types is referred to as a piezoelectric inkjet head. Such a piezoelectric inkjet head includes a pressure generation chamber of which wall partially includes a thin vibration plate. A piezoelectric element (piezo element) that is a pressure generating element is placed relative to the vibration plate. Applying a voltage to the piezoelectric element deforms the vibration plate with the deformation of the piezoelectric element. The deformation varies the pressure in the pressure generation chamber. The variation discharges the ink droplets.
Another type is a bubble jet (registered trademark) inkjet head in which a heating element is placed in the pressure generation chamber such that a current passes through the heating element to discharge the ink droplets. Such an inkjet head generates bubbles by heating a heating body by the passage of the current through the heating element. The pressure of the bubbles discharges the ink droplets.
Additionally, an electrostatic inkjet head is known as another type. The electrostatic inkjet head includes a vibration plate that forms a wall of the pressure generation chamber, and an individual electrode placed relative to the vibration plate and outside the pressure generation chamber so as to apply an electric field between the vibration plate and the individual electrode. The inkjet head deforms the vibration plate by the electrostatic force generated by the application of the electric field between the vibration plate and the individual electrode. The deformation varies the pressure and volume of the pressure generation chamber. The variation discharges the ink droplets from the nozzle.
FIG. 25 is a view illustrating the ink droplet discharged by the discharge pulse in an inkjet head using a piezoelectric element.
When the discharge pulse discharges an ink droplet, the ink droplet does not form a sphere just after being discharged by the discharge pulse. In other words, the ink droplet flies for a given period of time while forming a liquid column as illustrated. The liquid column separates from a nozzle after tens of microseconds have elapsed since the timing at which the discharge of the ink droplet by the discharge pulse has started.
A velocity difference occurs between the front part of the liquid column that is a main ink droplet (main droplet) and the rear part of the liquid column of the ink droplet due to the effect of the decay of the residual vibration of the ink in the pressure generation chamber. The rear part of the liquid column separated from the nozzle flies at a velocity lower than that of the main droplet. Thus, the rear part divides into a plurality of small droplets and formed into a plurality of satellite droplets.
The satellite droplets adhere to positions different from the main droplet on the recording medium. This causes the deterioration of image quality.
Additionally, such satellite droplets, which are divided small droplets, easily float (become mist) because the air resistance reduces the velocity of the satellite droplets. This causes a problem that the generated mist stains the nozzle surface of the head, the recording medium, or the inside of the printer.
To solve the problem, a method for driving an inkjet head is known. The method eliminates the velocity difference between the main droplet and the satellite droplets by applying an excitation pulse that increases the velocity of the satellite droplets at the timing just after the discharge pulse and synchronized with the residual vibration of the ink in the pressure generation chamber.
FIGS. 26A to 26D illustrate the excitation pulse that accelerates the velocity of the ink droplet that is the satellite droplets (namely, the excitation pulse for preventing satellite droplets, and hereinafter referred to merely as a excitation pulse S) and the temporal variations in vibration velocity of the ink near the nozzle (hereinafter, referred to as ink vibration velocity) in a conventional liquid discharge head although the conventional liquid discharge head is not described in Patent Document.
FIG. 26A illustrates an exemplary excitation pulse S in a conventional liquid discharge head. The voltage is shown on the vertical axis, and the time is shown on the horizontal axis. FIG. 26B illustrates the vibration velocity of the ink near the nozzle. The vibration is generated by the application of the discharge pulse illustrated in FIG. 26A to the piezoelectric element.
In that case, the excitation pulse S increases (accelerates) the velocity of the rear part of the liquid forming into a liquid column and flying from the nozzle. The excitation pulse S prevents the generation of the satellite droplets flying at a low velocity or mist by accelerating the velocity of the rear part of the liquid formed in a liquid column.
The ink droplet is discharged in the following manner. A voltage (VH-VL) is applied to the piezoelectric element to which a constant voltage VH is applied at the timing Tf illustrated in FIG. 26A to expand the pressure generation chamber. This temporality pulls the meniscus into the nozzle. A voltage (VL-VH) is applied to the piezoelectric element at the timing at which the meniscus is pulled into the pressure generation chamber most deeply to contract the pressure generation chamber. This pushes the ink in the pressure generation chamber to the outside.
The vibration velocity of the ink near the nozzle decays while vibrating with a natural oscillation period Tc of the pressure generation chamber after the discharge of the ink droplet. In that case, a pseudo vibration velocity of the ink near the nozzle is illustrated with a sine wave.
The ink near the nozzle sticks to the ink droplet pushed to the outside at the timing Tr illustrated in FIG. 26B and a liquid column extends from the nozzle during some periods of the vibration velocity of the ink from the timing Tr (for example, (a) to (d) in FIG. 25). The rear part of the liquid column (the part near the nozzle) decelerates while receiving a force that pulls the liquid column toward the inside of the nozzle with the decay of the vibration velocity of the ink near the nozzle and in the ink chamber.
When the force that pulls the liquid column toward the inside of the nozzle and the force that discharges the ink exceed the surface tension of the ink forming the liquid column, the liquid column separates from the ink droplet near the nozzle. Thus, the longer the time until the liquid column separates from the ink droplet is, the more the rear part of the liquid column decelerates.
Consequently, when the liquid column separates from the nozzle, the rear end of the liquid column (the rear part that separates and becomes the satellite droplets) flies at a velocity lower than that of the preceding main ink droplet (the main droplet) of the liquid column. Then, the satellite droplets adhere to positions away from the main droplet on the recording medium, or become mist.
In light of the foregoing, to prevent the deceleration of the rear end of the liquid column, the excitation pulse S illustrated in FIG. 26A is applied to the piezoelectric element of the liquid discharge head at the timing synchronized with the discharge pulse in a conventional apparatus. In other words, the time on the horizontal axis in FIG. 26A corresponds to the time on the horizontal axis in FIG. 26B. The excitation pulse S slightly expands the pressure generation chamber at the timing tf, and then slightly contracts the pressure generation chamber at the timing tr. In that case, the pulse width (tr-tf) of the excitation pulse S is ½ of the natural oscillation period Tc of the pressure generation chamber in length. An interval T1 from the timing Tr at which the application of the discharge pulse is completed to the timing tf at which the excitation pulse S starts is also ½ of the natural oscillation period Tc of the pressure generation chamber in length.
This accelerates the velocity of the rear part of the liquid column more than the velocity without the application of the excitation pulse S. This prevents the velocity of the satellite droplets from decelerating, and separates the liquid column from the nozzle faster.
A voltage (VH-VSL) of the excitation pulse S illustrated in FIG. 26A is lower than the voltage (VH-VL) of the discharge pulse. In addition to the excitation pulse S in FIG. 26A, an excitation pulse S having a voltage (VH-VL) identical to the voltage of the discharge pulse and having a pulse width extremely narrowed to prevent the ink droplet from being discharged as illustrated in FIG. 26C is used. An excitation pulse S that contracts and then expands the ink chamber and that is illustrated in FIG. 26D is also used. Intervals T1, each from the discharge pulse application completion timing Tr to the excitation pulse S, are ¾×Tc and TC as illustrated in FIGS. 26C and 26D, respectively.
Note that the excitation pulse S illustrated in FIG. 26C has the interval T1 of ¾×Tc. The excitation pulse S narrower than the pulse width of the discharge pulse expands and contracts the pressure generation chamber at that timing. This excites the residual vibration of the ink.
The discharge pulse that is a single pulse has been described above. However, when an ink droplet is discharged by a plurality of discharge pulses, the residual vibration of the ink in the pressure generation chamber after the discharge of the droplet is the overlap of the residual vibrations of the ink by the discharge pulses.
FIG. 25 illustrates that the ink droplets by two discharge pulses coalesce while flying ((a) to (d) in FIG. 25). In that case, adjusting the interval between the two discharge pulses accelerates the velocity of the ink droplet by the discharge pulse after the two discharge pulses.
However, the excitation pulse S is not applied at the optimal timing if the adjustment of the interval between the two discharge pulses produces the phase shift between the composite residual vibration of the ink in the pressure generation chamber and by the two discharge pulses, and the residual vibration of the ink and by the discharge pulse after the two discharge pulses, and the interval between the discharge pulse after the two discharge pulses and the excitation pulse S is the interval T1 described above.
In other words, the application of the excitation pulse S in the inkjet head at the conventional timing sometimes fails to eliminate the velocity difference between the main droplet and the satellite droplets.
Furthermore, if the ink has a low viscosity, the residual vibration of the ink in the pressure generation chamber decays slowly. Thus, the effect of the residual vibration of the last ink greatly remains when the next ink droplet is discharged. This causes a problem that the phase shift further increases.
Japanese Laid-open Patent Publication No. 2007-055147 discloses a liquid droplet discharge apparatus configured to apply, after the discharge pulse, an amplification pulse (the excitation pulse S) that amplifies the residual vibration after the application of the discharge pulse in order to prevent the generation of satellite droplets flying at a low velocity or mist.
However, the liquid droplet discharge apparatus is also configured to apply an amplification pulse to the residual vibration by a single discharge pulse without consideration of the phase shift by the residual vibrations that are overlaid after the ink droplets are discharged by a plurality of discharge pulses.
In light of the conventional problems, there is a need to prevent satellite droplets from reducing image quality or mist from staining the inside of the printer by preventing the generation of satellite droplets flying at a low velocity or the mist when a plurality of discharge pulses generates an ink droplet.